2017 NA
2016 NA
2015 NA
2014 NA
2013 NA
2012 NA
2011 NA
2010 NA
2009 NA
2008 20.7079234972678
2007 21.3471232876712
2006 21.3268493150685
2005 21.3260273972603
2004 21.2866120218579
2003 21.4416438356164
2002 21.1761643835616
2001 20.9493112947658